Pneumatic tire with reinforcement rubber layer

ABSTRACT

A pneumatic tire allowed to increase steering stability and durability while reducing rolling resistance, wherein a reinforced rubber layer extending from a bead part along a side wall part is installed in the side wall part. A rubber composition forming the reinforced rubber layer has a loss of tangent (tan δ) measured at 60° C. of 0.01 to 0.25, a JIS-A hardness measured at 23° C. of 70 to 95, and a breaking elongation in a tension test measured at 23° C. of 200% or more.

TECHNICAL FIELD

The present invention relates to a pneumatic tire provided with areinforcement rubber layer having a high degree of hardness, thereinforcement rubber layer extending from a bead portion along asidewall portion. More specifically, the present invention relates to apneumatic tire allowed to increase driving stability and durabilitywhile being allowed to reduce rolling resistance.

BACKGROUND ART

In a pneumatic tire, in order to exert excellent driving stability byimproving casing stiffness, what is normally performed is to bury,inside a sidewall portion, a reinforcement rubber extending from a beadportion to the sidewall portion. However, if the sidewall portion ismade thinner for the purpose of making a tire lighter, the reinforcementrubber layer has to be inevitably made thinner. As a result, that leadsto a reduction in stiffness of the tire. To complement the reduction instiffness of the type, it is required that the reinforcement rubberlayer be arranged to reach the highest possible position. A variety ofrelated arts for that purpose have been proposed (refer to PatentDocuments 1 to 5, for example).

-   Patent Document 1: Japanese patent application Kokai publication No.    Hei8-318713-   Patent Document 2: Japanese patent application Kokai publication No.    Hei8-318714-   Patent Document 3: Japanese patent application Kokai publication No.    Hei9-300923-   Patent Document 4: Japanese patent application Kokai publication No.    Hei11-28916-   Patent Document 5: Japanese patent application Kokai publication No.    2001-71715

However, since a large amount of deformation occurs in a portion from awidest-width position of the tire to a shoulder portion of the tire whenthe tire is run, if rubber having a high degree of hardness, which hasbeen conventionally used, is used as it is, there are such problems as:reduced durability of the tire, because of a small breaking elongation;and deteriorated rolling resistance, because of a large hysteresis loss.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a pneumatic tireallowed to increase driving stability and durability while being allowedto reduce rolling resistance.

A pneumatic tire according to the present invention for achieving theabove object is a pneumatic tire provided with a reinforcement rubberlayer extending from a bead portion along a sidewall portion, inside thesidewall portion. The pneumatic tire is characterized in that a rubbercomposition forming the reinforcement rubber layer has a loss tangent(tan δ) of 0.01 to 0.25 when measured at a temperature of 60° C., aJIS-A hardness of 70 to 95 when measured at a temperature of 23° C., anda breaking elongation not less than 200% when measured at a temperatureof 23° C. in a tensile test.

If physical properties of the reinforcement rubber layer extending fromthe bead portion along the sidewall portion are thus defined, it becomespossible to improve driving stability and durability while rollingresistance being reduced even in a case where the sidewall portion ismade thinner for the purpose of making a tire lighter.

In the present invention, it is desirable that the reinforcement rubberlayer extend from the bead portion along the sidewall portion, andbeyond the widest-width position of the tire, reaches the shoulderportion of the tire. In addition, in order for the above physicalproperties to be exhibited, a rubber composition forming thereinforcement rubber layer is desired to be one formed in a manner that20 to 120 weight parts of silica, and 0 to 60 weight parts of carbonblack as an arbitrary element, are mixed with 100 weight parts ofrubber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half cross-sectional view taken along a meridian, showing apneumatic tire according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a detailed description will be given of a configuration ofthe present invention with reference to the attached drawings.

FIG. 1 shows a pneumatic tire according to an embodiment of the presentinvention. In FIG. 1, reference numerals 1, 2, and 3 respectively denotea tread portion, a sidewall portion, and a bead portion. A carcass layer4 is mounted between a pair of the left and right bead portions 3, 3,and has an end portion folded back from the inside to the outside of thetire around a bead core 5. A plurality of belt layers 6, 6 are buried onthe outer periphery of the carcass layer 4. These belt layers 6, 6 arearranged in order that cords of the respective belt layers can tilt tothe circumferential direction of the tire and that the cords can crossone another between the layers.

Inside the sidewall portion 2, a reinforcement rubber layer 7 extendingfrom the bead portion 3 along the sidewall portion 2 is buried, thereinforcement rubber layer 7 having a high degree of hardness. Thereinforcement rubber layer 7 extends from the bead portion 3 along thesidewall portion 3, and beyond the widest-width position of the tire,reaches the shoulder portion of the tire. In the cross-sectional viewalong a meridian of the tire, the lowest-end part of the reinforcementrubber layer 7 forms a substantially triangular shape, the lowest-endpart being relatively close to the bead portion. The highest-end part ofthe reinforcement rubber layer 7 has a uniform thickness which isthinner than the substantially triangular shape, the highest-end partbeing relatively close to the shoulder portion. That is, thereinforcement rubber layer 7 enables the sidewall portion 2 to bethinner while reinforcing the sidewall portion 2.

A rubber composition forming the above reinforcement rubber layer 7 hasa loss tangent (tan δ) of 0.01 to 0.25 when measured at a temperature of60° C., a JIS-A hardness of 70 to 95 when measured at a temperature of23° C., and a breaking elongation not less than 200% when measured at atemperature of 23° C. in a tensile test.

As a rubber composition exhibiting the above physical properties, it ispossible to adopt one formed in a manner that 20 to 120 weight parts ofsilica and 0 to 60 weight parts of carbon black are mixed with 100weight parts of rubber. In a case where a mixed amount of silica is outof the above range, it is difficult to obtain the above physicalproperties. In a case where carbon black is additionally mixed, itbecomes difficult to obtain the above physical properties if a mixedamount of carbon black exceeds 60 weight parts.

As silica, for example, dry method white carbon, wet method whitecarbon, colloidal silica, and precipitated silica can be cited. Thesekinds of silica may be used by one or in combination of two or more.

As base rubber, for example, natural rubber (NR), styrene-butadienecopolymer rubber (SBR) can be cited. These kinds of rubber may be usedby one or in combination of two or more. Additionally, to the rubbercomposition thus obtained, in addition to silica and carbon black, acompounding agent used regularly may be added. As compounding agents,for example, process oil, a vulcanizer, a vulcanization accelerator, anantioxidant, and a plasticizer can be cited.

The pneumatic type configured as above is provided with thereinforcement rubber layer 7, which extends from the bead portion 3along the sidewall portion 2, and beyond the widest-width position ofthe tire, reaches the shoulder portion of the tire, inside the sidewallportion 2. Therefore, even in a case where the sidewall portion 2 ismade thinner for the purpose of making the tire lighter, casingstiffness of the tire is high and driving stability is excellent.

Furthermore, even though the tire assumes such a configuration that thereinforcement rubber layer 7 extends beyond the widest-width position ofthe tire and reaches the shoulder portion of the tire, since thereinforcement rubber layer 7 is formed by using rubber of a low losstangent, rolling resistance of the tire can be reduced. In addition,since the rubber composition forming the reinforcement rubber layer 7has a large breaking elongation, durability of the tire can besufficiently satisfactory.

Here, the rubber composition forming the reinforcement rubber layer 7 ismade to have a loss tangent (tan δ) of 0.01 to 0.25 at a temperature of60° C., with the reasons being: that it is technically difficult to makeit to have a loss tangent less than 0.01; and that rolling resistance isincreased if the loss tangent exceeds 0.25. A more desirable range ofthe loss tangent is 0.07 to 0.25. Note that the loss tangent (tan δ)here is the one measured by using a viscoelastic spectrometer(manufactured by Toyo Seiki Seisaku-sho, Ltd.), with conditions of afrequency of 20 Hz, an initial distortion of 10%, and a dynamicdistortion of plus or minus 2%.

Additionally, the rubber composition forming the reinforcement rubberlayer 7 is made to have a JIS-A hardness of 70 to 95 when measured at atemperature of 23° C. This is because: if the JIS-A hardness is lessthan 70, a sufficient effect of improving driving stability cannot beobtained due to insufficient casing stiffness of the tire; and on theother hand, if the JIS-A hardness exceeds 95, riding comfort anddurability are deteriorated.

Moreover, the rubber composition forming the reinforcement rubber layer7 is made to have a breaking elongation not less than 200% when measuredat a temperature of 23° C. in a tensile test. This is because if thebreaking elongation is less than 200%, durability of the tire becomesinsufficient. Although it is not required to particularly limit an upperlimit of the breaking elongation, the upper limit is about 350% inreality. Note that the breaking elongation here is the one measured incompliance with JIS K6251.

In the aforementioned embodiment, the reinforcement rubber layer buriedin the sidewall portion has a configuration where: its part relativelyclose to the bead portion assumes a substantially triangular shape, itspart relatively close to the shoulder portion assumes a sheet-likeshape, and the two foregoing parts continuously extend. In the presentinvention, however, it is not required to particularly limitcross-sectional shapes of the reinforcement rubber layer. Additionally,the part relatively close to the bead portion and the part relativelyclose to the shoulder portion may be separated. In the case they areseparated, it is also possible that while the part relatively close tothe bead portion is covered by a turned-up portion of the carcass layer,the part relatively close to the shoulder portion is arranged to theoutside of the turned-up portion of the carcass layer.

While the detailed description has been given of the preferredembodiment of the present invention hereinabove, it should be understoodthat various modifications to, substitutions for, and replacements withthe preferred embodiment can be carried out as long as the modificationsto, the substitutions for, and the replacements do not depart from thespirit and the scope of the present invention defined by the attachedclaims.

Hereinbelow, descriptions will be given of results of experiment onpneumatic tires which were actually manufactured. First, as rubbercompositions to form a bead filler, rubber compositions A to Irespectively composed of blends shown in Table 1 below were prepared.Note that the rubber compositions A to I were prepared in the followingmanner. Rubber and a compounding agent such as carbon black are mixedfor five minutes by using a Banbury mixer which is an internal mixer,and a vulcanization accelerator and sulfur are mixed, on an open rollmill, with the resultant of the foregoing mix.

TABLE 1 Rubber composition A B C D E F G H I Natural rubber *1 70 70 7070 70 70 70 70 70 SBR *2 30 30 30 30 30 30 30 30 30 Carbon black *3 9060 50 70 55 40 50 60 — Silica *4 — — 40 25 10 65 45 35 95 Silanecuppling agent *5 — — 4 2 1 6 4 3 9 Zinc oxide *6 7 7 7 7 7 7 7 7 7Stearic acid *7 2 2 2 2 2 2 2 2 2 Antioxidant *8 1 1 1 1 1 1 1 1 1Vulcanization accelerator *9 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Sulfur*10 7 7 7 5 7 5 5 5 5 Loss tangent (tan δ) (at 60° C.) 0.3 0.2 0.2 0.270.19 0.2 0.21 0.24 0.15 Degree of hardness (at 23° C.) 85 68 85 85 70 9585 86 85 Breaking elongation (at 23° C.) 180 250 190 210 208 200 203 213220 In Table 1, *1: RSS#3, *2: Nipol 1502 manufactured by ZEONCorporation, *3: DIA-HA manufactured by Mitsubishi Chemical Corporation*4: Nipsil AQ manufactured by Nippon Silica Industrial *5: Si-69manufactured by Degussa AG *6: Zinc oxide No. 3 manufactured by SeidoChemical Industry *7: manufactured by NOF Corporation *8: SANTOFLEX 6PPDmanufactured by FLEXSIS *9: Nocceler CZ manufactured by Ouchi ShinkoChemical Industrial *10: manufactured by Karuizawa Refinery

Here, pneumatic tires with a tire size of 205/65R17 were manufactured.The pneumatic tires were provided with the same structure except thatdifferent kinds of bead filler were used as a reinforcement rubber layerburied in the sidewall portion. In the conventional example, a fillerheight is set to be a normal height (45 mm from the highest end of thebead core in a radial direction of the tire). In the comparativeexamples 1 to 4 and in the examples 1 to 5, heights of filler (80 mmfrom the highest end of the bead core in a radial direction of the tire)is set to be higher than the normal height, and kinds of filler rubberare made different.

With regards to these test tires, rolling resistance, durability andcasing stiffness are evaluated as follows, and the results thereof areshown in Table 2.

Rolling Resistance:

Each of the test tires was mounted onto an wheel of a rim size of 15×6½JJ, and rolling resistance thereof was measured by setting the tire in astate having an air pressure of 190 kPa, driven by a speed of 80 km/h,and having a load of 4.6 kN, by using a drum-type tire testing machine.Results of the assessment are shown in index numbers where a rollingresistance value of the conventional example is taken as 100. A highervalue of the index number means that rolling resistance is higher.

Durability:

Each of the test tires was mounted onto an wheel of a rim size of 15×6½JJ, and a mileage until the tire was destroyed was measured by settingthe tire in a state having an air pressure was 190 kPa, driven by aspeed of 80 km/h, and having a load as described below, by using adrum-type tire testing machine. The load was started with 88% of themaximum load and was increased by 13% each step. Note that one stepcorresponded to two hours until the load reached 140% of the maximumload, and to four hours after the load reached 140% of the maximum load.Results of the assessment are expressed in index numbers where adurability value of the conventional example is taken as 100. A highervalue of the index number means that durability is more excellent.

Casing Stiffness:

A lateral spring constant of each of the test tires was measured.Results of the assessment are expressed in index numbers where a casingstiffness value of the conventional example is taken as 100. A highervalue of the index number means that casing stiffness is higher, andtherefore driving stability is more excellent.

TABLE 2 Conventional Comparative Comparative Comparative Comparativeexample example 1 example 2 example 3 example 4 Example 1 Example 2Example 3 Example 4 Example 5 Filler height Normal High High High HighHigh High High High High filler filler filler filler filler fillerfiller filler filler Filler rubber A A B C D E F G H I Rollingresistance 100 125 95 94 103 93 92 94 100 90 Durability 100 90 105 95105 104 101 102 105 108 Casing 100 110 105 115 115 110 130 115 115 115stiffness

As apparent from Table 2, in the comparative example 1, although casingstiffness was increased due to the bead filler higher than that of theconventional example, rolling resistance was increased and durabilitywas reduced. In the comparative example 2, an effect of increasing thecasing stiffness was insufficient as a result of an insufficienthardness of the rubber composition forming the bead filler. In thecomparative example 3, durability was reduced as a result of aninsufficient breaking elongation of the rubber composition forming thebead filler. In the comparative example 4, rolling resistance wasincreased as a result of a high loss tangent of the rubber compositionforming the bead filler.

On the other hand, in all of the examples 1 to 5, since the respectiverubber compositions forming the bead filler possess the requiredphysical properties, it was possible that the casing stiffness wasincreased and the durability was improved while the rolling resistancewas reduced.

INDUSTRIAL APPLICABILITY

This invention can be effectively used in the tire manufacturingindustry, and additionally in the automobile manufacturing industry.

1. A pneumatic tire provided with a reinforcement rubber layer extendingfrom a bead portion along a sidewall portion, inside the sidewallportion thereof, wherein: a rubber composition forming the entirereinforcement rubber layer has a loss tangent (tan δ) of 0.01 to 0.25when measured at a temperature of 60° C., a JIS-A hardness of 70 to 95when measured at a temperature of 23° C., and a breaking elongation notless than 200% when measured at a temperature of 23° C. in a tensiletest, the reinforcement rubber layer extends from a bead core at thebead portion along the sidewall portion, and beyond a widest-widthposition of the tire, and reaches a shoulder portion of the tire, therubber composition forming the reinforcement rubber layer is one formedin a manner that 20 to 120 weight parts of silica, and 0 to 60 weightparts of carbon black are mixed with 100 weight parts of rubber, and thetotal of the weight parts of silica and carbon black in the rubbercomposition is between 65 and 105.